Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Meteograms

To display meteograms

Fig8.1.4-1: To view meteograms:

1. On charts page, click ensemble Meteograms.
2. Select meteogram type from drop-down menu or display all. ensemble meteograms by clicking on square icon.
   
3. Select location by name or Lat/Long.

Fig8.1.4-2: Alternative way to view meteograms:

1. On Forecast Charts and Data page, click on any Forecast Range.  A menu of available charts appears.
   
2. Select Medium Range and Point Based Products.  A selection of products appears.
3. Select the Meteograms display.
4. Select meteogram type from drop-down menu or display all ensemble diagrams by clicking on square icon.
   
5. Select location by name or Lat/Long.
   

View a meteogram example.

Overview

The ensemble meteogram provides a probabilistic interpretation of the ensemble for specific locations.  It displays the time evolution of the distribution of several meteorological parameters from the ensemble by a box and whisker plot.  All ensemble meteograms have a title section, giving the name (unless overwritten by the user), the true height of the chosen location, and the co-ordinates of the grid point used based on the ensemble resolution.   

The sub-section “Selection of grid points for Meteograms” explains the method of interpolation of grid point forecast data for presentation for a given location.

Box and Whisker Plot

Forecast distributions are displayed using a box and whisker plot (see Fig8.1.4-3) which shows the median (short horizontal line), the 25th and 75th percentiles (wide vertical box), 10th and 90th percentiles (narrower boxes) and the minimum and maximum values (vertical lines).

 Fig8.1.4-3: The box and whisker plot used in the ECMWF 10- and 15-day ensemble meteograms.

Ensemble meteograms are available for:

• 15-day medium-range plot at 6hr intervals for:
  • weather (total cloud cover, 10m wind strength, 6hr rainfall total, 2m temperature).
  • waves (significant wave height, mean wave direction, mean wave period, and strength and distribution of direction of the 10m wind).  If no sea grid point nearby, only wind data is plotted from the nearest land grid point and the wave diagrams are empty. 
• 15-day extended-range plot at 24hr intervals for:
  • weather only (mean total cloud cover, 24hr rainfall total, mean strength and distribution of direction of the 10m wind, max and min 2m temperature).
  • weather only (as above) but also showing M-climate.

Note:

• The Ensemble Control Forecast (ex-HRES) values are shown as a blue line on the 10-day ensemble meteogram. 
• The 15-day ensemble meteogram displays the probability distribution for each calendar day from 00UTC to 00UTC.  For forecasts with data time of 12UTC the first and last 12 hours in the forecast period are excluded and only 14 (instead of 15) daily distributions are generated. 

10-day ensemble meteogram

Fig8.1.4-4: 10-day medium-range meteogram for Athens data time 00UTC 12 May 2017.  Solid blue lines are Ensemble Control Forecast (ex-HRES).  The red numbers above the precipitation panel are the greatest precipitation value reached by any ensemble member.  Ensemble extreme values cannot be ignored as the evolution of every ensemble member is considered to be equally probable.  Note: Forecast temperatures at 00UTC, 06UTC, 12UTC, 18UTC are shown. 15-day meteograms show forecast maximum and minimum temperatures.  UTC is used exclusively in the meteograms. 

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

15-day ensemble meteogram

Fig8.1.4-5: 15-day medium-range meteogram for Dublin from ensemble data time 12UTC 22 June 2023.  The displayed values are for the 24hr period each day, with additionally the distribution of 10m wind direction. Note: Forecast maximum and minimum temperatures are shown.  10-day meteograms show forecast temperatures at 00UTC, 06UTC, 12UTC, 18UTC.  UTC is used exclusively in the meteograms.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

15-day ensemble meteogram with M-climate

Fig8.1.4-6: As Fig8.1.4-5 with the addition of M-climate data.  M-climate data is shown by shaded colours with percentiles similar to the box and whisker scheme.  The temperature box and whisker for 24 June lies confidently above the 99th percentile of the M-climate.  The median wind forecast for 30 June lies above the M-climate values (above the 75th percentile of the M-climate) with the whisker extending above the 99th percentile of the M-climate.  The median precipitation for the 25 June lies between the 50th and 75th percentile.  Note: Forecast maximum and minimum temperatures are shown.  UTC is used exclusively in the meteograms.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.4-7: Illustration of the relationship between 10day ensemble presentation and 15day presentation (truncated to 10days for ease of comparison).

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.4-8: Illustration of the relationship between 10day ensemble presentation and 15day presentation (truncated to 10days for ease of comparison).

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Weather parameters in the ensemble meteograms

• Total cloud cover in the 10-day ensemble meteogram is the instantaneous forecast value in oktas (eighths of the sky covered by cloud).  In the 15-day extended ensemble meteogram it is the daily average of ensemble forecast values at 06, 12, 18 and 24UTC.  When all members have 0 cloudiness (clear sky) or 8 oktas cloudiness (overcast), there is no line or box at all.  Note:
  • When the forecast is very uncertain and all cloud amounts are more or less equally likely, the columns cover almost the whole range from 0 to 8 oktas, which can be wrongly interpreted as ““overcast””.
  • An alternative display (currently only available within the ecCharts meteogram platform) has a circle divided clockwise into eight arcs, each arc representing 1/8 cloud cover. So, for example, the arc covering 45°-90° represents 2/8 cloud cover. The shading within each arc is proportional to the number of members that forecast this particular degree of cloud cover or more.
  • See also interpretation of total cloud cover.

• Total precipitation in the 10-day ensemble meteogram is the accumulated precipitation (sum of convective and large-scale) over 6hr periods (00-06UTC, 06-12UTC, etc).  In the 15-day meteogram it is the accumulated precipitation over 24hr periods (00-24UTC).  Note:

  • On the 10-day meteogram the box-and-whisker plot locations align with the end of the 6 hour period.

  • Probabilities for intervals longer than the 6hr and 24hr time intervals cannot be deduced from the ensemble meteogram.

  • Periods of probabilities >0% in every interval can be wrongly interpreted as uninterrupted rain.

  • Consideration of the median alone can be wrongly interpreted as protracted dry spells.

  • The precipitation shown on the ensemble meteograms cannot be directly inter-compared as the rainfall range (y-axis) varies from one location to the next and from one forecast to the next.  The rainfall range is chosen separately for each ensemble meteogram so that 100% of the predicted values are covered for the 15-day ensemble meteograms, and at least 90% of the predicted values are covered for the 10-day ensemble meteograms (if the top of the distribution is beyond the scale maximum the largest 6-hourly total is shown at the top as red numbers).

• 10m wind speed in the 10-day ensemble meteogram is the instantaneous forecast value in m/s.  Note this is the mean speed, not the diagnosed gust.  In the 15-day ensemble meteogram as it is the 24-hour wind-speed average of ensemble forecast values at 06, 12, 18 and 24UTC.  Note:
  
  • The peaks of the whiskers should not be interpreted as wind gusts.  Ensemble products related to gusts should be used (e.g. CDF diagrams).
    
    
• 10m wind direction (only shown in the 15-day ensemble meteogram) is the daily distribution of directions obtained by taking each 6-hourly forecast step for the day (50 members x 4 forecast steps at 06-12-18-24UTC) and allocating it to the relevant octant.  The area of an octant is proportional to the probability of that wind direction (i.e. to the proportion of forecasts falling in that octant).  The probability of each octant is shown by shading light (low) to dark (high).  Note:
  • The wind roses shown on the ensemble meteograms cannot be directly compared as each is scaled to the size of the most populated octant. The size of the wind rose does not refer to wind speed.
    
    
• 2m temperature  in the 10- day ensemble meteogram is shown as instantaneous forecast values at 6-hourly intervals.  In the 15-day ensemble meteogram it is shown as daily maximum and minimum temperatures (in °C).  Note:
  • The forecast temperature is adjusted by using a 5.5K/km lapse rate applied across the difference between the station height (as displayed in the title) and the ensemble orography (the relative heights of ensemble and true orography are shown in the top right corner of the meteogram web page.  In some instances, some information is also included in the temperature panel title of the meteogram itself).

At longer lead times, the ensemble mean and the ensemble median will tend to gravitate asymptotically towards the M-climate.  This is most clearly seen when the first ten days of the forecast are anomalous (e.g. after an initial spell of cold and rainy weather, the ensemble tends to indicate a return to milder and drier conditions at longer forecast ranges).  This follows logically from the fact that at an infinite range, when predictive skill is completely lost, a climatological value constitutes the optimal forecast.

Interpreting ensemble meteograms

It is necessary to assess critically the parameters shown on ensemble meteograms. 

• Bias: Verification of previous forecasts, particularly recent forecasts within a similar meteorological regime, may allow an insight into the bias of the latest forecast. 
• Bi-modal distribution of forecast results:  Occasionally ensemble forecasts diverge into a bi-modal (or possibly multi-modal) distribution in two (or more) distinct patterns.  This might happen if there were model uncertainty regarding timing or positioning of a cold front.  So for a given location a number of ensemble members may show warm midday temperatures while others show much cooler temperatures.   Bi-modal distribution of forecast results will not be shown by meteograms.  Box-and whisker plots cannot do this - the effect would be just to stretch out the boxes.  However bimodal distributions can be apparent on plume diagrams.
• Snow/rain discrimination:  If a majority of ensemble members forecast temperatures below 0°C and, at the same time, a large number of members forecast substantial precipitation, there is no way to determine the likelihood of snowfall from the standard meteogram diagram alone.  It could be that the precipitating members might all have temperatures well above 0°C.   The ecCharts meteogram product shows ensemble probabilities of precipitation types by category.  Probability of combined events can only be calculated from the original ensemble data.  Several charts of combined probabilities are available on ecCharts.
• Relative spread of forecast results:  This may vary considerably between one parameter and another in the same forecast step.  For example: 
  • In a high-pressure blocking event, there might be a small spread in precipitation and wind, but a large spread in temperature and cloudiness.
  • in a zonal regime, there might be a large spread in the precipitation and wind and a small spread in the temperature and cloudiness.
• Severe weather events:  The ensemble can only predict severe weather events of the kind that the can resolved by the current resolution (~9km).   Forecaster experience and local knowledge should help identify the severity and persistence of smaller scale active storms. 

Interpretation of total cloud cover

The top row of the standard 10-day meteogram shows the ensemble total cloud cover in box and whisker format.  High total cloud cover on this diagram can give an impression of gloomy or dull conditions whereas the cloud layers may not be uniformly thick or extensive.   A better idea of the structure of the model cloud layers may be obtained from meteograms of high, medium and low cloud cover.  These are available by selecting "views" then "meteogram windows" on ecCharts.  This allows assessment of the impact of each cloud layer, and in particular whether the full total cloud cover is made up from:

• large amounts of cloud at all levels implying dull overcast conditions.
• large amounts of cloud at one or two levels implying less gloomy conditions.
• large amounts of high cloud alone implying brighter conditions.  

However, users should also refer to the vertical profiles to decide the thickness of each layer.  Thin high cloud can allow quite bright skies while thick high cloud can give a very gloomy day.

A better forecast can be obtained by assessing the cloud forecast meteograms and charts a little more deeply.

Fig8.1.4-9: Example of a frontal canopy moving eastwards over St Andrews, East Scotland.  DT 00UTC 15 Mar 2024, VT 12UTC 16 March 2024 (indicated by dashed red line).  The total cloud is 8/8 cover and an initial impression is for grey dull conditions.  The other meteograms show:

• little low cloud (mostly <2/8 cover, low probability 5/8 cover, mean amount near 0/8 cover).
• little medium cloud (mostly <1/8 cover, low probability 7/8 cover, mean amount near 0/8).
• most cover is at high levels (8/8 cover, low probability 1/8 cover, mean cover near 8/8).

The total cloud cover in the ensemble is mostly 8 oktas, the low and middle levels have small amounts of cloud.  Therefore a brighter sky may be expected than by considering the total cloud cover alone.   However, the vertical profile strongly suggests thick upper cloud layers implying greyer skies which is reasonable ahead of an approaching front.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.4.10: Example of frontal canopy moving eastwards over Pamiers, South France.  DT 00UTC 15 Mar 2024, VT 12UTC 16 March 2024 (indicated by dashed red line).  The total cloud cover is high and an initial impression is for grey dull conditions.  The other meteograms show:

• no low cloud.
• no medium cloud.
• most cover is at high levels (8/8 cover, low probability 1/8 cover, mean cover near 8/8).

The total cloud cover in the ensemble is mostly 8 oktas, the low and middle levels have no cloud.  The vertical profile strongly suggests only thin upper cloud layers.  Therefore a much brighter sky may be expected than by considering the total cloud cover alone.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Coastal and mountainous regions

When creating a meteogram for a specific location, the land-sea mask at the four surrounding ensemble grid points is considered.

• If there is at least one land grid point within these four, then the nearest land point will be chosen and the meteogram title section shows "ENS Land Point" together with its location and ensemble altitude.
• If only sea points are available then the nearest sea grid point will be chosen and the meteogram title section shows "ENS Sea Point" together with its location and ensemble altitude of 0m.

Data at the selected ensemble point is calculated using HTESSEL and FLake according to the proportions of land and sea cover within the surrounding grid point box (see examples below, or the Land-Sea Mask section for details).

Some influences of the adjacent sea areas or mountains may be over- or under-represented by the ensemble meteograms.  This can significantly affect the forecast parameter on the meteogram (temperature, wind, etc).    Users should assess differences in meteograms for coastal, island or mountainous regions.   In particular consider:

• the impact of the grid point(s) relative to the land-sea mask.
• the variation of the altitude of the land.   Forecast values at the grid point nearest to the location are adjusted for altitude using a standard lapse rate assumption.  The difference in temperature can be considerable.

Note: the so-called land-sea mask processing (where the land or sea nature of the source and target points was used to adjust the interpolation weights) used by the old ECMWF interpolation software scheme (called EMOSLIB) is not used by default in the new MIR interpolation package that was introduced early in 2019.

(FUG Associated with Cy49r1)